Having been traditionally perceived as a diagnostic modality, ultrasound is rapidly emerging as a most promising therapeutic tool for non-invasive ablation of cancerous and other tissues, for enhanced drug delivery, and for a range of other therapeutic applications that include thrombolysis, opening of the blood-brain barrier, tendon and bone repair, tissue erosion, vaccine delivery and acoustic haemostasis. In all of these applications, ultrasound-induced bubble activity (acoustic cavitation) has been found to play a major role in enhancing several desirable bioeffects (heating, cell permeability, drug diffusion lengthscales, etc). The term ‘cavitation’ is used hereafter to encompass all possible bubble behaviours in an ultrasound field, including transient or inertial cavitation; stable cavitation including shape oscillations of the bubble wall; and the response of thermally stabilized bubbles (such as boiling bubbles) in an ultrasound field. The process of cavitation itself could have been initiated through spontaneous, acoustically driven nucleation, or through the injection of stabilized gas bodies such as ultrasound contrast agents, or of solid microparticles that are designed with appropriate surface characteristics (hydrophobicity and surface roughness) to facilitate cavitation inception.
Cavitation is an inherently unstable phenomenon and, once initiated in the body (which is by itself quite unpredictable), tends to decay rapidly whilst the associated bubble cloud readily shifts positions. Being unable to sustain cavitation activity at the desired location for prolonged periods of time means that the potential benefits of cavitation cannot be fully exploited.